Numerous electromagnetic sensors or probes have already been proposed. Nevertheless, presently-known means do not always give full satisfaction.
In particular, it has not been possible until now to make probes of small size that are nevertheless capable of covering a broad measurement band: whatever solutions have been envisaged in known systems, any reduction in size (typically to less than one-quarter of the wavelength) is synonymous with reducing the passband.
In an attempt to mitigate that drawback, proposals have indeed been made to develop probes based on frequency-selective printed antennas by adding an active electronic circuit that compensates for said selectivity as a function of frequency. For that purpose, non-linear elements are associated with the antenna. Unfortunately, that solution puts a considerable limit on sensitivity and therefore makes it difficult to extract performance at a precise frequency.